Conventionally, in digital copying machines, scanners, etc., various processes are applied to an image signal obtained by reading a document by means of a CCD (Charge Coupled Device) image sensor to attain an improved quality of a recorded image (copied image of a document).
Generally known methods of improving an image quality of the recorded image include the method of applying an optimal density correction to the image signal obtained by reading the document in accordance with the features of the document. According to such image processing, for example, in the case of the document like newspaper, etc., a density correction is applied so as to remove the background, or the lack of hiding, and in the case of the document written by a pencil, a density correction is applied so as to thicken thin characters.
The described method of correcting the density of an image signal is adopted, for example, in the background density detecting device of Japanese Examined Patent Application No. 55796/1989 (Tokukohei 1-55796 (hereinafter referred to as Gazette (1)), wherein a runlength at each image density is obtained, the image density which maximizes the result of addition is determined to be the background density, and a density of an image signal is corrected based on the background density.
Another method of correcting the density of an image signal is adopted in the image processing device of Japanese Unexamined Patent Application No. 236277/1993 (Tokukaihei 5-236277) (hereinafter referred to as a Gazette (2)), wherein a histogram of an electric signal representing the document is prepared, and the features of the document are identified based on the features of the histogram (brightest level, darkest level, maximum frequency, and maximum frequency level) so as to covert the signal level of the electric signal based on the conversion table prepared in accordance with the information indicative of the described features.
A still another method of correcting the density of an image signal is disclosed in an image processing apparatus of Japanese Unexamined Patent Application No. 189128/1994 (Tokukaihei 6-189128) (hereinafter referred as Gazette (3)), wherein a coordinate of the colored portion of the document is specified by coordinate input means, and the image of the document is read by an image input apparatus, the density of the image of the document at the specified coordinate is detected by the background density detection means, the threshold value, based on which the background region is removed, is set based on the detected density, and the background is removed based on the threshold value, thereby correcting the density of the image signal.
However, according to the density correction method adopted in the background density detection device of Gazette (1), the image density which maximizes the result of addition of the runlength is determined to be the background density, and the background region is removed. Therefore, for example, in the case of a reversal document including outline characters in which a character region is in white and a background region is in black, a photographic document, etc., the black background region which maximizes the image density is removed. As this makes a difference in density between the character region and the background region smaller, a quality of the recorded image may be lowered. Therefore, the described density correction method is not applicable to various types of documents such as the described reversal document, etc.
On the other hand, when the density correction method adopted in the image processing apparatus of Gazette (2) is applied to the digital copying machine, a preliminarily scanning of the document is required when reading the image data to prepare a histogram of the document, thereby presenting the problem that the image processing speed is lowered. In order to identify the document with accuracy without applying the preliminarily scanning, a memory device of a large capacity is required for recording the image data of the document.
Additionally, in Gazette (2), the document type is identified by processing the feature data of the document based on the threshold value, and a density conversion table is prepared in accordance with the type of the document. However, the described method of processing based on only the threshold value does not permit the type of the document to be identified with accuracy, and it is difficult to prepare the density conversion table reflecting the characteristics of the feature data of the document in detail.
Thus, the method of the Gazette (2) does not permit the documents of various kinds to be identified with accuracy and the density correction to be performed with accuracy in accordance with the characteristics of the document, thereby presenting the problem that the quality of the recorded image is lowered depending on the type of the document.
According to the density correction method adopted in the image processing apparatus of the Gazette (3), as the process of specifying the background region is required by specifying the coordinate of the document, in the case of copying documents of various kinds, the process of specifying the coordinate is required for each document, thereby presenting the problem that an overall time required for processing the entire image becomes long.